1. Field of the Invention
The present invention relates generally to a multi-hop relay system. More particularly, the present invention relates to an apparatus and method for buffering packets in a multi-hop relay system supporting hop-by-hop retransmission.
2. Description of the Related Art
For the purpose of increasing data rate and expanding cell coverage in a cellular network, techniques for improving the propagation environment of users located at a cell boundary or in a shadowing area by use of relay nodes have been proposed. In the proposed techniques, a Base Station (BS) services users directly under a good channel environment and via an adjacent relay node under a poor channel environment, to thereby achieve an overall improved communication quality.
When packet loss occurs on a radio link in a relay cellular network, the BS or its upper Base Station Controller (BSC) retransmits a lost packet toward a Mobile Station (MS), for packet recovery, as with a legacy cellular network. However, even though a packet is lost between the BS and a relay node, it is the MS that sends a request for packet retransmission to the BS via the relay node after detecting the packet loss and then receives a retransmitted packet via the relay node. As a consequence, a time delay is increased in packet recovery and a radio link is wasted. On the other hand, if the BS or the relay node takes charge of reliable packet transmission to the next node (i.e. a relay node or an MS), the next node detects packet loss on the link, requests retransmission to the upper node, and receives a retransmitted packet, thus saving time in packet recovery and avoiding a waste of the radio link. Herein, the former method is called end-to-end retransmission and the latter method is called hop-by-hop retransmission.
In a handoff procedure of a typical cellular network, an MS determines whether to initiate the handoff procedure based on the channel statuses between the MS and a serving BS and between the MS and neighbor BSs. When determining to start the handoff procedure, the MS requests the serving BS to approve a handoff to a target BS and, in turn, the serving BS requests the target BS to approve the handoff of the MS. The target BS determines whether to approve the handoff request according to its resource status and responds to the MS via the serving BS. If the response is approval, the serving BS transfers a context of the MS, i.e. session information and buffered data, to the target BS and reroutes the data transmission path of the MS to the target BS. Thus, the handoff of the MS is completed.
The above handoff procedure is also applicable to the relay cellular network except that when a handoff occurs to an MS served by a relay node, communications are not conducted between a serving BS and the MS not via a direct link but via the relay node. Also, packets buffered in the relay node as well as packets buffered in the serving BS should be transferred to a target BS before a context transfer.
As illustrated in FIG. 1, when a serving relay node 103 services an MS 101 in a relay network supporting hop-by-hop retransmission, some packets are buffered in a serving BS 105, while other packets are buffered in the serving relay node 103. When the MS 101 starts a handoff, the packets buffered in both the serving BS 105 and the serving relay node 103 should be transferred to a target BS 107 before a context transfer. The packet transfer takes place in three steps. In step (1), the serving relay node 103 transfers packets buffered in a relay buffer 130 to the serving BS 105. Then, the serving BS 105 transfers the received buffered packets to a target BS buffer 170 of the target BS 107 in step (2). In step (3), the serving BS 105 transfers packets buffered in a serving BS buffer 150 to the target BS buffer 170 of the target BS 107. While steps (2) and (3) cause a short transmission delay and a low transmission cost because the packets are transmitted over wired networks, the packet transfer on a radio link in step (1) results in a long time delay and a high transmission cost. This problem becomes more serious when a plurality of relay nodes are involved in servicing the MS.